Performance analysis and design of a torsional vibration isolator using a negative stiffness device

Performance analysis and design of a torsional vibration isolator using a negative stiffness device
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In mechanical systems, rotating shafts are the most important device. The vibrations related the rotating shaft shorten lifespan, reduce operating efficiency, and cause noise. So, the vibration isolation has been studied for a long time. A method of isolating vibration is to connect a vibration isolation device between an input part and an output part. The vibration isolators are classified into active vibration isolators that utilize sensors and actuators to control the system and passive vibration isolators that utilize structural characteristics. The active vibration isolator is better than the passive one in the respect of performance. For the financial reasons, however, it is sometimes necessary to use the passive vibration isolation device. The disadvantage of passive one is that the effect of the vibration isolation only appears when the frequency ratio is above a certain size. Therefore, it is necessary to reduce the natural frequency of the system to cut off the wider range of the excitation frequencies. To reduce the natural frequency of the system, the stiffness or mass of the system must be changed. However, this method has a limitation in improving the insulation performance because it affects the design. To overcome this problem, a nonlinear vibration isolator is proposed. In this paper, the nonlinear stiffness of the isolator is realized by using the negative stiffness device. By the way, the meaning of negative stiffness used in this paper is the structural characteristic, not the material one. The meaning of negative stiffness is that the displacement occurs in the opposite direction of the force. However, these materials do not exist in real. All the materials constituting the negative stiffens device have a positive stiffness, but negative stiffness is achieved by the displacement of the system by structurally positioning the components. The negative stiffness device allows the stiffness of the entire system to change according to the state of motion. Particularly, when the stiffness of the system becomes close to zero, which is referred to as a quasi-stiffness characteristic. The research using the negative stiffness device focused only on the isolation of the vibration in the translational direction, but recently, a model applying the principle of the negative stiffness to isolate the torsional vibration has proposed. In a recent study about the torsional vibration, however, there is a limitation that only the vibration isolation performance of the negative stiffness device was confirmed qualitatively. Therefore, not only torsional vibration insulation performance but also power transmission performance should be considered. To apply the proposed model to the actual design, it is necessary to quantitatively represent the performance of the proposed model. For this purpose, the vibration isolation performance and the power transmission performance of the negative stiffness device were defined as indices. In addition, the equation of motion of the proposed model was derived and changed the form to the dimensionless. We conducted a frequency sweep to check the vibration isolation performance for various excitation frequencies. As a result, the vibration isolation index according to frequency was confirmed. However, as a result of the dimensionless parameter study, it was confirmed that the vibration insulation performance greatly changes according to the magnitude of the input moment. This is because the system stiffness varies depending on the magnitude of the input moment in the steady. Therefore, in the latter part of this paper, we introduced a model consist of multiple negative stiffness devices to satisfy the robustness for variable the input moment. By using a multiple negative stiffness model, the isolation performance of the system can be improved even if the magnitude of the input moment is changed.
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